1. Field of the Invention
The present invention relates to an image signal processing apparatus and, more particularly, to an image encoding apparatus having a wavelet transforming unit, and an image decoding apparatus having an inverse wavelet transforming unit.
2. Description of the Background Art
In an image encoding apparatus of the JPEG 2000 system, by performing a DC level shifting process and a color space converting process, a two-dimensional image signal is obtained. To extract a desired image region from the two-dimensional image signals, a coordinate system called a reference grid is used. Concretely, by designating offset values in the row and column directions from the origin of the reference grid, a desired image region is specified.
In the JPEG 2000 system, in DWT, the order of processes of using a vertical filter first, and then a horizontal filter is determined. FIG. 9 is a diagram showing a state where a two-dimensional filter is applied on signals of a two-dimensional image of 16 rows and 16 columns in order of a vertical filter and a horizontal filter. The left part shows a state after the vertical filter is applied, and the right part shows a state after the horizontal filter is applied after that.
As shown in the left part of FIG. 9, by applying the vertical filter, vertical components in even-numbered rows become low-pass data L, and vertical components in odd-numbered rows become high-pass data H. As shown in the right part of FIG. 9, by further applying the horizontal filter, horizontal components in even-numbered columns become low-pass data L, and horizontal components in odd-numbered columns become high-pass data H. In the right part in FIG. 9, (LL) shows that both of the horizontal and vertical components are L, (HL) shows that the horizontal component is H and the vertical component is L, (LH) shows that the horizontal component is L and the vertical component is H, and (HH) indicates that both of the horizontal and vertical components are H. The arrow X indicates the vertical direction, and the arrow Y indicates the horizontal direction.
In the case where an offset value from the reference grid is not designated, a process starts from the origin of the reference grid (the left upper corner of the two-dimensional image signal). In this case, an input/output process is executed in order of a low-pass filter and a high-pass filter in each of the vertical and horizontal filters. On the other hand, when the offset value from the reference grid is designated to an arbitrary value, the low-pass filter is not always executed first in each of the vertical and horizontal filters. According to the designated offset value, the order of executing input/output of the low-pass filter and the high-pass filter has to be controlled.